The disclosure of Japanese Patent Application No. 2001-328900 filed on Oct. 26, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a method of controlling a current applied to an electromagnetically driven valve such as an intake valve and an exhaust valve of an internal combustion engine and a control system.
2. Description of Related Art
Conventionally, operation of an internal combustion engine is controlled by opening and closing an intake valve or an exhaust valve by a cam of a camshaft synchronously driven by a crankshaft. With recent progress of computer-controlled operation in the field of internal combustion engines, operation of an internal combustion engine is increasingly controlled by opening and closing an intake valve or an exhaust valve by an electromagnetic actuator. The use of the electromagnetic actuator enables the valve to be opened and closed at various timings. As a result, various methods of controlling the operation of an internal combustion engine (especially for a vehicle) have been proposed.
Basically, in the intake valve or the exhaust valve of an internal combustion engine, a disc-like armature is attached to a valve shaft, and a pair of electromagnets for opening and closing the electromagnetically driven valve are disposed so as to be apart from the armature by a distance equivalent to a sum of an opening/closing stroke of the valve and a thickness of the armature, respectively such that those valves face with each other. The valve is opened by applying electric current to the electromagnet for opening the valve (hereinafter referred to as the valve opening electromagnet) so as to attract the armature thereto. The valve is closed by applying electric current to the electromagnet for closing the valve (hereinafter referred to as the valve closing electromagnet) so as to attract the armature thereto. In general, as the armature is formed from a paramagnetic material such as a soft iron, each of the valve opening and valve closing electromagnets generates only an attraction force to attract the armature, not a repulsion force. As the magnetic attraction force is inversely proportional to the square of the distance, and the valve opening/closing stroke is relatively long, a size of the valve opening electromagnet, thus, has to be increased to generate the attraction force sufficient to attract the armature released from the valve closing. Likewise a size of the valve closing electromagnet has to be increased to generate the attraction force sufficient to attract the armature released from the valve opening electromagnet to the valve closing electromagnet. The electromagnetically driven intake valve or exhaust valve as aforementioned is provided with a pair of springs for urging the armature at a neutral position of the valve opening/closing stroke. More specifically, one of the pair of springs, serving as the valve opening spring, forces the armature released from the valve closing electromagnet toward the direction away from the valve closing electromagnet. The other spring, serving as the valve closing spring, forces the armature released from the valve opening electromagnet toward the direction away from the valve opening electromagnet. The aforementioned structure forms a vibration system in which a valve element including the armature, the valve shaft to which the armature is attached, and the valve body is suspended between those springs. The valve can be opened and closed by adjusting the current applied to the valve opening electromagnet and the valve closing electromagnet using resonance of the vibration system. Therefore, there is no need to increase each size of those electromagnets. Assuming that the displacement of the valve body or the armature resulting from the valve opening/closing operation is expressed as a lift amount, the lift amount is correlated with the moving speed of the valve body or the armature as shown in FIG. 1. In order to establish the relation as shown in FIG. 1, the current applied to the electromagnets may be correlated with the lift amount as shown in FIG. 2. As a result, the lift amount changes with times as shown by a solid line of FIG. 3. In the case where the electromagnet has a small capacity, and the valve movement goes out of the timing at which the resonance of the vibration system can be used for assisting the valve operation, the valve may be stuck at the neutral position of the valve opening/closing stroke. The aforementioned stuck state is typically known as xe2x80x9cstep-outxe2x80x9d.
Japanese Patent Laid-Open Publication 11-294209 discloses alternate application of current to the first and the second electromagnets synchronously with a natural period of the vibration system to gradually increase the amplitude of the armature so as to recover the valve from the step-out state. Generally reverse current is applied to the electromagnet to which application of current is interrupted so as to extinguish the residual magnetic field upon switching the opening/closing operation of the electromagnetically driven valve. Then the inductance of the valve against the reverse current is detected. If the detected inductance is smaller than a predetermined value, it is determined that the electromagnet has failed to attract the armature and, thus, the valve has been brought into the step-out state.
According to the aforementioned method, it is determined whether the armature has been attracted to the electromagnet on the basis of the inductance of the electromagnet that releases the armature against the reverse current applied thereto for extinguishing the residual magnetic field. If the inductance is smaller than a predetermined value, it is determined that the armature has not been attracted to the electromagnet and, thus, it is determined that the step-out has occurred. The valve may be recovered from the step-out by taking a required procedure so as to avoid any failure in operating the internal combustion engine owing to the step-out.
However, according to the aforementioned method, the step-out of the valve is generally detected with a delay corresponding to a period taken for operating a cycle of opening or closing the valve.
According to the aforementioned method in which the step-out is regarded as the state where the valve is stuck at a neutral position between the valve opening/closing stroke. However, there may be a case in which the valve body fails to open/close in accordance with the normal timing even if the valve is held at the closing or the opening position. Such behavior of the valve that indicates possibility of the step-out may be recovered to the normal condition after performing at least one cycle of the valve opening/closing operation.
It is an object of the invention to provide a method of controlling current applied to an electromagnetically driven valve and a control system so as to detect a failure in the valve as well as to recover the detected failure at the appropriate timing.
In a control method of current applied to an electromagnetically driven valve including a first electromagnet, a second electromagnet, an armature that is attracted by one of the first and the second electromagnets after being released from attraction of the other electromagnet so as to move a valve body of the electromagnetically driven valve between a valve opening position and a valve closing position, and a spring that holds the armature at a neutral position between a position where the armature is attracted by the first electromagnet and a position where the armature is attracted by the second electromagnet, it is determined whether a displacement of the armature is equal to or smaller than a predetermined threshold value after an elapse of a predetermined time from start of switching operation of the electromagnetically driven valve between an opening state and a closing state. When it is determined that the displacement is equal to or smaller than the predetermined threshold value, application of the current to the electromagnetically driven valve is changed so as to be different from application of current performed when it is determined that the displacement is equal to or greater than the predetermined value.
In an embodiment and drawings of the invention, it is assumed that the displacement of the valve body or the armature includes both movement for opening the valve and movement for closing the valve in opposite directions. It is also assumed that the displacement direction is expressed as being positive either in the opening or closing direction, and the displacement amount also takes a positive value measured either in the opening or closing direction.
The current application may be changed by interrupting application of the current to one of the first and the second electromagnets that is to attract the armature.
Alternatively, the current application may be changed by applying the current to one of the first and the second electromagnets that has released the armature so as to be attracted by the other electromagnet.
In a control method of current applied to electromagnetically driven valve including a first electromagnet, a second electromagnet, an armature that is attracted by one of the first and the second electromagnets after being released from attraction of the other electromagnet so as to move a valve element of the electromagnetically driven valve between a valve opening position and a valve closing position, and a spring that holds the armature at a neutral position between a position where the armature is attracted by the first electromagnet and a position where the armature is attracted by the second electromagnet, it is determined whether a displacement of the armature is equal to or smaller than a predetermined threshold value after an elapse of a predetermined time from start of switching operation of the electromagnetically driven valve between an opening state and a closing state. An abnormality in one of the opening sate and the closing state of the electromagnetically driven valve is detected when it is determined that the displacement is equal to or smaller than the threshold value.
In case of performing an open control to the electromagnetically driven intake valve of an internal combustion engine, current may be applied from the intake valve to an electromagnetically driven exhaust valve in the same cylinder so as to be closed. In case of performing a closing control to the electromagnetically driven exhaust valve of an internal combustion engine, current may be applied from the exhaust valve to an electromagnetically driven intake valve in the same cylinder so as to be closed.
The step-out may be caused by the failure in the electromagnet or the electric system. However, it is mainly caused by the structure for driving the intake or the exhaust valve, in which the valve element including the armature, valve body, and the shaft member for connecting the armature to the valve body is slidably guided by a guiding device of a bearing type. There may occur a sliding failure in the sliding portion owing to the frictional resistance. Further the lubricating oil enters the gap between the armature and the surface of the valve opening or the valve closing electromagnet on which the armature is attracted. This may form an oil film along the surface through which the armature and the electromagnet contact therebetween when the armature is attracted to the electromagnet. Therefore, the operation of releasing the armature from the electromagnet is hindered by the resistance caused by the surface tension of the oil film. The frictional resistance rapidly reduces as the sliding starts from static friction with the maximum value to the dynamic friction. Accordingly, those factors such as friction or surface tension that cause the step-out of the valve may be found at the beginning of the opening/closing operation of the electromagnetically driven valve. If those factors are detected at the beginning of the opening/closing operation, the operation delay corresponding to the time taken for 1 cycle of the valve opening/closing time may be eliminated.
The step-out caused by the aforementioned factors may be detected simultaneously with its occurrence. This makes it possible to provide the recovering procedure immediately so as to minimize the failure in operating the internal combustion engine.
The resistance owing to the friction or the oil film as described above rapidly reduces once the valve element including the valve body, the armature and the shaft member connecting the armature to the valve starts moving from one of the closed or opened position to the other position. The valve element may be stuck at the beginning of application of the reverse current to the electromagnet to which the armature has been held for extinguishing the residual magnetic field. However, the valve element can be recovered from the stuck state and start moving at the end of the reverse current application. The valve opening/closing control may be changed when it is determined that the displacement of the armature fails to reach a predetermined value even after an elapse of a predetermined time from start of switching valve opening/closing operation. This makes it possible to perform normal opening/closing of the valve even if its valve element is stuck and then starts moving with a certain delay so long as the delay is within a recoverable range.
Application of current to one of the first and the second electromagnets that is to attract the armature may be interrupted, that is, the valve opening/closing control may be discontinued, when the displacement of the armature fails to reach a predetermined threshold value even after an elapse of a predetermined time from start of switching the valve opening/closing operation. Alternatively, current may be applied to the electromagnet that has released the armature from attraction. This makes it possible to prevent the valve from stepping out owing to the stuck state of the valve element and to allow the valve to be normally operated within a predetermined number of valve opening/closing cycles.